Hologram information recording is a system for recording/reproduction of information as two-dimensional page data and it can increase the recording density and transmission speed sharply compared with a bit data system such as DVD. For this reason, constant efforts are being made on research and development of hologram information recording as one of the promising next-generation information recording systems.
In particular, an optical information recording medium (hereinafter also referred to as hologram recording medium) using a volume phase hologram recording material (hereinafter also referred to as hologram recording material) shows a diffraction efficiency of 1, the theoretical maximum, and allows overwriting (multiple recording) of information. Hence, there are great expectations for arrival of the time when this medium is put into practical use for high-density recording. Photopolymers are frequently used as hologram recording materials in consideration of simplicity of manufacture and diversity in selection of raw materials
When the recording layer of a hologram recording medium containing radically photopolymerizable components and a radical photoinitiator (the recording layer of this kind may hereinafter also be referred to as hologram recording layer) is irradiated simultaneously with a reference beam and an information beam, both composed of a coherent active energy ray, a light-and-dark interference pattern is created and this induces polymerization reaction to occur in the light area and causes the polymerizable components to diffuse in the direction to diminish and eliminate the concentration gradient generated by the polymerization reaction, that is, the polymerizable components diffuse from the dark area to the light area of the interference pattern. On the other hand, the non-polymerizable components diffuse in the direction to compensate for the diffused polymerizable components, that is, in the reverse direction from the light area to the dark area of the interference pattern. Thus, the components are distributed by concentration in the hologram recording layer corresponding to the light intensity of the interference pattern and this is recorded as a modulation structure of the refractive index.
The recording capacity of a hologram recording medium is proportional to the thickness of a hologram recording layer in principle. Therefore, it is advantageous to make a hologram recording layer as thick as possible within the allowable range, that is, to the extent that the influence of such factors as light absorption by the materials and volume shrinkage accompanying the polymerization reaction on the recording performance is substantially tolerated. The thickness of a hologram recording layer demanded in reality ranges from approximately 200 μm to 2 mm and this is extremely thick in comparison with that of the conventional light information recording media.
The patent document 1 discloses a substantially solid photopolymerizable composition that forms an image of refractive index by exposure to a chemically active radiation as the only treating step and essentially comprises 25-75% of (a) a solvent-soluble, thermoplastic polymer binder, 5-60% of (b) a liquid ethylenic monomer, and 0.1-10% of (c) a photopolymerization initiator that activates the polymerization of the said ethylenic monomer upon exposure to a chemically active radiation. A solvent is used here to dissolve the components uniformly and to lower the viscosity of the composition to such a degree as to allow easy application to a substrate. The composition is applied to a substrate and the solvent is then removed by evaporation in a drying step to form a hologram recording layer and the necessity of providing a drying step for evaporation and removal of the solvent has substantially limited the thickness of the recording layer to 100 μm or less.
Several disclosures have been made on hologram recording materials that can form a relatively thick hologram recording layer of approximately 200 μm or more without need of solvents, hologram recording media using the said materials, and the methods of manufacture. For example, some of them disclose a three-dimensional crosslinked polymer matrix formed in-situ during the formation of a hologram recording layer (the patent documents 2-7, the non-patent document 1, and elsewhere).
The aforementioned three-dimensional crosslinked polymer matrix is considered to play multiple roles of providing a hologram recording material with enough strength to maintain the proper shape as a hologram recording layer, suppressing excessive migration of polymerizable compounds, and reducing the volume shrinkage accompanying the polymerization in the course of hologram recording (the non-patent document 1).
The patent document 2 discloses an optical article wherein the product consists of a three-dimensional crosslinked polymer matrix and one kind or more of photoreactive monomers, at least one kind of photoreactive monomer contains a monomeric functional group and additionally a portion which is substantially non-existent in the polymer matrix, and the matrix polymer is compatible with a polymer formed by the polymerization of one kind or more of photoreactive monomers. The patent document 2 further discloses an optical product wherein a three-dimensional crosslinked polymer matrix is formed in the presence of the aforementioned one kind or more of photoreactive monomers by a polymerization reaction that is independent of the polymerization of the said one kind or more of photoreactive monomers.
The hologram recording medium of the aforementioned composition requires no solvent in the formation of a hologram recording layer and it has an advantageous ability to form a layer with a thickness in the range of several hundreds of micrometers to several millimeters relatively easily.
High transparency is required for a hologram recording medium. Therefore, the three-dimensional crosslinked polymer matrix is required to be compatible with a polymerizable monomer and with a polymer formed therefrom.
However, only a limited number of matrix polymer-monomer combinations satisfy this requirement of compatibility as far as the optical products disclosed in the patent document 2 are concerned. Moreover, even a combination showing satisfactory compatibility may have a problem in that the difference in refractive index between the matrix polymer and the monomer or between the matrix polymer and the polymer formed from the monomer cannot be made sufficiently large.
The data recorded as an interference pattern in hologram information recording are fixed completely by a treatment such as post-exposure. In continuous recording of a large volume of data, a long time is required from the beginning of recording of data to the completion of fixing of the recorded data and the data once recorded may deteriorate in the meantime. Therefore, a hologram recording medium is required to retain the data recorded in whatever time required from the continuous recording to the fixing treatment intact (this property is hereinafter referred to as record retention property).
However, the problem with the optical products disclosed in the patent document 2 is their insufficient record retention properties.
The patent document 3 discloses a volume phase hologram recording material that comprises a three-dimensional crosslinked polymer matrix having a plurality of reactive groups and an ability to record interference fringes created by the interference of a coherent light utilizing the difference in refractive index and has no polymerizable monomer as a constituent to record a hologram.
The patent document 4 discloses a photosensitive composition for volume phase hologram recording which comprises (a) a compound having one or more active methylene groups in the molecule or a compound having two or more active methylene groups in the molecule, (b) a compound having two or more groups in the molecule to which an active methylene group or a carbanion formed therefrom adds nucleophilically, (c) a catalyst for the Michael reaction, (d) a photopolymerizable compound, and (e) a photopolymerization initiator.
The hologram recording materials disclosed in the patent documents 3 and 4 show improved record retention properties, but they are still insufficient in sensitivity.
As described above, a number of materials containing a three-dimensional crosslinked polymer matrix to be produced in the step for forming a hologram recording layer are disclosed as hologram recording materials, but no material of high sensitivity, high contrast, and good record retention properties has been offered yet.
Patent document 1: JPH2-3081 A
Patent document 2: JPH11-352303 A
Patent document 3: WO2005/078531
Patent document 4: JP2005-275389 A
Patent document 5: JP2004-287138 A
Patent document 6: JP2005-250060 A
Patent document 7: WO2005/78532
Non-patent document 1: T. J. Trentler, J. B. Boyd, and V. L. Colvin, Epoxy Resin-Photopolymer Composites for Volume Holography, Chemistry of Materials, Vol. 12, pp. 1431-1438 (2000)